Open Sonic the Hedgehog

Open Sonic is a freeware, open-source platformer inspired by the venerable Sonic games of the nineties, but with some differences in gameplay. While not merely being a clone of the old games, the main objective is still the same: to transverse the whole level as fast as possible, collecting rings and killing the enemies by jumping on them. The basic mechanics are unaltered, the main addition being the cooperative play more akin to Knuckles Chaotix, where the player has to change characters to open passages and go to places that Sonic can't reach. Besides the blue hedgehog, there is "Tails", a yellow fox that can fly by rotating his two tails, and Knuckles, a red echidna that can climb walls. The levels have teleporters at certain points to summon all the characters to a place and avoid tedious backtracking. The game text can be changed to several languages, new levels can be downloaded from the official site, and a level editor is included to create custom stages.